29. The 46th Mersenne Prime

A Mersenne number is a positive number that can be expressed in the form 2n-1. A Mersenne prime is a Mersenne number that is, well, prime. Searching for higher and higher Mersenne primes is the unofficial national sport of mathematicians. The 45th and 46th (right) Mersenne primes were found this year, the latter by a team at UCLA. It has almost 13 million digits.